While playing in Adventure mode while on my multi-player server, I died while fighting a skeleton. When I went back to collect my things, I was unable to punch the skeleton for any damage, nor were any hits registering on the skeleton (it did not flash red or recoil from the hit).

After several minutes, I came back to that area and was able to punch, so it may have been a temporary problem. The skeleton was able to shoot me, and killed me very quickly.

I have a video where this is visible, as I was recording a Let's Play when it happened. It is at http://www.youtube.com/watch?v=EuSBNQ4cukg time is 11:28 (before death) to 14:14, where I was able to damage the skeleton (not sure if it was the same one, though).